Significance Analysis of mutants in the transmembrane of env (env-TM) domain of the pathogenic SIVmac239 clone may provide insight on the role of this domain in viral load and simian AIDS (SAIDS) pathogenesis in infected macaques. Information obtained from such in vivo studies will facilitate the development of live-attenuated lentivirus vaccines, and may also identify a novel target(s) for antiviral therapy. Objectives A large portion of the env-TM domain of SIV is dispensible for viral replication in vitro in tissue culture cells. The main objective is to determine the importance of this domain in vivo by inoculating rhesus macaques with viruses containing point mutations in intracellular portion of the env-TM of SIVmac239. Results Mutations (missense and stop codons) in the env-TM of SIVmac239 reduced replication of the respective virus in cultures of peripheral blood lymphocytes from macaques. Four macaques were inoculated with a viral clone containing a premature stop codon in env-TM. In the acute phase of infection, these animals are exhibiting high virus load. Thus, either rapid reversion of the stop codon has occurred, or the intracellular portion of env-TM is not required for high virus levels. Future Directions Virus will be recovered from macaques infected with the mutant clone, and the env gene will be sequenced to detect potential reversions in env-TM. An additional viral mutant, containing several point mutations and a deletion in env-TM, will be tested in macaques for virus load and (potential) disease progression. Biochemical experiments will be performed to identify a cellular protein(s) that interacts with the intracellular portion of env-TM. KEYWORDS SIV, AIDS, antiviral therapy